1. Field of the Invention
The invention relates to a system for the stabilization of an object with reference to a moving platform, including a stabilization device provided with servo motors and a servo control unit. A platform is connected to a side of the stabilization device to be stabilized.
2. Discussion of the Background
The invention is in particular suitable for the stabilization of sensors on a naval ship. A system of this type is known inter alia from patent specification U.S. Pat. No. 5,573,218, which is hereby incorporated by reference. The stabilization device described in the present patent specification comprises a stack of wedge-shaped elements, whereby each element is rotatable with reference to the underlying element. The orientation of the object is thus realized by the relative rotation of sub-elements which are mutually rotatable about non-parallel shafts. The servo control unit ensures that the object, through rotation of the individual elements, assumes and maintains a desired, usually horizontal, position. In the embodiment described in this patent specification, the position of the object with reference to a substructure is calculated on the basis of the geometry of the stabilization device and the relative angular position of each element with reference to its underlying element. These relative angular positions are determined with the aid of encoders. A drawback of the stabilization device is that vibrations may cause stabilization errors owing to a certain measure of elasticity inherent in the stabilization system. This may have adverse consequences, particularly if the stabilization device is to be used for stabilizing an optical scanner, which requires a high degree of stabilization accuracy.
A possible solution to this problem may be obtained from patent specification U.S. Pat. No. 3,358,285. The stabilization platform described therein is provided with gyroscopes in order to determine its position with respect to an inertial coordinate system. However, this solution has the drawback that angle-indicating gyroscopes are relatively costly, certainly if a high degree of accuracy is required. Besides, no benefit can then be derived from the fact that naval ships are usually equipped with a central gyro system, used for determining the ship""s angular positions with respect to an inertial coordinate system.
The system according to the invention has for its object to provide a solution to these problems and is characterized in that:
the system further includes an angular velocity measuring device with respect to an inertial coordinate system, which device is connected to the stabilization platform;
the servo control unit is provided with means to connect the servo control unit to an external angular position measuring unit of the moving platform;
the servo control unit is designed to control the servo motors on the basis of angular position signals from the angular position measuring unit and angular velocity signals from the angular velocity measuring device.
A naval ship usually includes a system of centrally situated gyroscopic angular position encoders used for measuring the ship""s angles with respect to a north horizontal coordinate system. Sensors that require stability, are positioned at a certain distance to the angular position encoders. Ship deformations and vibrations may cause the measured angular position and the actual angular position at the sensor to deviate. A solution to this problem is obtained by combining the angular velocity signals with the angular position measurements.
This yields two different measuring instruments to realize the stabilization, each having its own specific advantages and drawbacks. Actual practice will show that one of the two instruments performs a more accurate measurement in a certain frequency range than in another frequency range.
A favorable embodiment is thereto characterized in that the calculation means for calculating the control signals are designed to perform a frequency-dependent weighting of the angular velocity signals and the angular position signals.
The angular velocity indicators are preferably used for stabilization in a high frequency band while the angular position indicators are suitable for stabilization in a low frequency band. Thus, ship vibrations are not transmitted to the stabilization platform, whereas high-frequency disturbances (abrupt motions) are suppressed to a sufficient extent.
A further favorable embodiment is thereto characterized in that the calculation means are designed to perform the frequency-dependent weighting such that the control signals for frequencies below a certain frequency xcfx890 are substantially determined by the angular position signals and the control signals for higher frequencies are substantially determined by the angular velocity signals.